Rafik Hariri, Lebanon's former prime minister, was assassinated in a huge bomb attack yesterday that killed at least nine others, injured more than a hundred and threatened to shatter the country's fragile peace.

The murder of the billionaire opponent of Syria's influence in Lebanon raised the spectre of a return to violence 15 years after the civil war.

His killing was condemned by world leaders and drew a sharp response from the White House, which directed pointed remarks towards Syria. The blast left a five-metre deep crater on Beirut's seafront, where restaurants were serving lunch. It tore apart the armour-plated vehicles of Mr Hariri's motorcade and broke windows up to a mile away.

A previously unknown group calling itself Support and Jihad in Syria and Lebanon claimed to have carried out the bombing, calling the attack the first of a campaign of "martyrdom attacks" aimed at "infidels, renegades and tyrants".

In a video aired on the al-Jazeera television network, a bearded man in a turban read a statement on behalf of the group, describing the killing as "just punishment" for Mr Hariri's close ties to the Saudi government.

According to the Associated Press, Lebanese authorities identified the man as Ahmed Abu Adas, a Palestinian who lives in Beirut's western district of Tariq al-Jadidah.

Security officials, speaking on condition of anonymity to AP, said he left his house early yesterday and never came back.

They said he was suspected of having links with al-Qaida. A computer and other equipment were confiscated from his house, the officials added.

It was not clear whether the explosion was a suicide bomb, nor whether the explosives (estimated as the equivalent of 300kg of TNT) had been packed in a car, on the side of the street, or hidden under the road.

The assassination threatened to raise tensions across the Middle East, where a Palestinian-Israeli ceasefire last week had given hope of more peaceful times.

It could also worsen the fraught relations between Damascus and Washington, which believes the Syrian government is turning a blind eye to the flow of weapons and insurgents into Iraq and undermining efforts to bring peace to the region.

George Bush began his second term in the White House by warning Syria that he would confront it for "harbouring terrorists" within its borders and in Lebanon.

Yesterday, the White House came close to blaming Syria's presence in Lebanon for Mr Hariri's murder, describing the attack as "a terrible reminder that the Lebanese people must be able to pursue their aspirations _ free from violence, and intimidation and free from Syrian occupation".

"We hold the Lebanese authority and the Syrian authority, being the authority of tutelage in Lebanon, responsible for this crime _ " they said in a statement.

Mr Hariri, 60, a Sunni with dual Lebanese and Saudi Arabian citizenship, had been prime minister for a total of 10 years, resigning last October in protest at a constitutional amendment giving Emile Lahoud, the Syrian-backed president, another three years in office. He joined calls for Syria to pull its troops out of Lebanon and had been expected to draw support away from the pro-Syrian government in elections due to be held this spring.

President Lahoud described the killing as "a dark point in our national history", as the government called an emergency cabinet meeting late yesterday afternoon.

Lebanon's army command announced forces were put on maximum alert and that soldiers on holiday were being recalled..

In Sidon, Mr Hariri's home town, shops were shuttered. One group of people attacked a van with Syrian workers inside, shattering its windshields.

Jacques Chirac, the French president, called for an internationally supervised investigation, but in central Beirut most people were blaming Syria though other theories involving the Israelis and the CIA also surfaced.

The explosion tore through Mr Hariri's convoy just before 1pm as it was travelling down the Corniche, Beirut's famed coastal stretch of hotels and restaurants. Mr Hariri was pronounced dead on arrival at the city's American University hospital.

Local television showed footage of a man on fire in the front passenger seat of his car fumbling with the door before falling out on to the ground.

Several of Mr Hariri's bodyguards were also killed and his leading aide, Basil Fuleihan, a former economy minister, was reported to be in a critical condition.

Violence has been rare since the civil war ended 15 years ago and many Lebanese who had fled to Canada and the US have returned.